1. Field of the Invention
The present invention relates to the system employed and circuitry used with an ensemble of clocks to obtain an ensemble time. More particularly, the present invention relates to an improved algorithm defining ensemble time that can be, for example, implemented with Kalman filters for obtaining an improved estimate of time from an ensemble of clocks.
2. Description of the Related Art
For a number of years, groups of precision clocks used in combination have provided the "time" in situations in which high precision timekeeping is required. For example, an "official" time for the United States is provided by the atomic time scale at the National Bureau of Standards, the UTC(NBS), which depends upon an ensemble of continuously operating cesium clocks. The time interval known as the "second" has been defined in terms of the cesium atom by the General Conference of Weights and Measures to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. Other clocks may be calibrated according to this definition. Thus, while each clock in a group or ensemble of clocks is typically some type of atomic clock, each clock need not be a cesium clock.
Even though one such atomic clock alone is theoretically quite accurate, in many applications demanding high accuracy it is preferred that an ensemble of atomic clocks be used to keep time for a number of reasons. Typically, no two identical clocks will keep precisely the identical time. This is due to a number of factors, including differing frequencies, noise, frequency aging, etc. Further, such clocks are not 100% reliable; that is, they are subject to failure. Accordingly, by using an ensemble of clocks in combination, a more precise estimate of the time can be maintained.
When an ensemble of clocks is utilized to provide an estimate of time, various techniques may be employed for processing the signals output by the clocks to obtain the "time". Typically, interclock time comparisons are made to determine the relative time and frequency of each clock. The noise spectrum of each clock is represented by a mathematical model, with noise parameters determined by the behavior of the individual clock. Clock readings are combined based on these comparisons and models to produce the time scale.
A problem with known systems for providing an estimate of time or a time scale based upon an ensemble of clocks is that the individual states of the clocks are not observable and only clock differences can be measured. As a result, there are an infinite number of solutions or estimates of time possible. Stated another way, the systems are underdetermined. For example, in systems that implement a Kalman filter approach to estimating time, the lack of observability and resultant underdetermined character of the system manifest themselves in covariance matrix elements that grow on each cycle of the Kalman recursion. Since the computation is implemented on a computer system with finite accuracy, this growth eventually causes computational problems. In addition to reducing the consequences of lack of observabililty, it is also desirable to obtain a substantially unbiased estimate of clock performance that can be used to achieve a ensemble time system that is adaptive. Moreover, there is a need to achieve this adaptive quality while also maintaining or improving the robustness of the resulting ensemble time.
There is yet a further need for an ensemble time system that permits estimates of all of the spectral densities of the clocks in the system, detects steps in state estimates, and provides improved performance over a wider range of averaging times.
Based on the foregoing, there is a need for a system for estimating time based on a clock ensemble that addresses the aforementioned needs.